1. Field of the Invention
The present invention relates to a semiconductor circuit, and more particularly to a semiconductor circuit for embodying a post-synaptic neuron firing process in a biological synapse.
2. Description of the Related Art
The nervous system of a living body is consisted of numerous nerve cell neurons and synapses connecting neurons. Synapses may be classified as electrical or chemical, based on the signal transmission between neurons. The electrical synapses are found in invertebrates and myocardial cells etc. Since the others are known as the chemical synapses, hereinafter, word of “synapse(s)” indicates the chemical synapse(s).
Recently, many studies have been made to mimic the nervous system of a living body, in particular the brain nervous system, by a nerve-like circuit system (i.e., a neuromorphic computation system) using semiconductor devices.
By the way, in order to embody the neuromorphic computation system, a post-synaptic neuron firing process must be efficiently emulated.
For emulating the post-synaptic neuron firing process, the properties of the nervous system of a living body must be considered. Thus, with reference to FIGS. 1 and 2, the properties of the nervous system of a living body are briefly described.
One neuron 100, as shown in FIG. 1, is connected to a plurality of neurons 200 and 300 by synapses 400 through a plurality of dendrites 120.
Each neuron 100, 200 or 300 has basically a nucleus 110 in a cell body (a soma), and there are a plurality of dendrites 120 to receive a stimulated signal around the cell body and an axon 130 connected by an axon hillock 122 to transmit the stimulated signal to one side of the cell body.
The axon 130 generally has a length of about 10,000 times of diameter of the cell body, is wrapped with a plurality of myelin sheaths 132 interlaid with a node of Ranvier 134 and consists of axon collaterals 136 and axon terminals 138.
A synapse 400, as an enlarged view shown in FIG. 1, indicates a connecting region between two neurons, namely, a meeting region between an axon terminal of the pre-synaptic neuron 200 and a dendrite of the post-synaptic neuron 100 interlaid with the narrow space, as a synaptic cleft 402, of about 20 nm.
The transmission process of the synapse 400 is simply described as the followings with reference to the enlarged view shown in FIG. 1.
First, when a fire is triggered by a stimulation exceeded over the threshold value (Vth, an about −55 mV) in the pre-synaptic neuron 200, the stimulation as an electrical signal is transmitted to the axon terminal through the axon with the repeat of depolarization and repolarization by alternately opening and closing sodium 202 and potassium (not shown) channels, respectively.
The stimulation transmitted to the axon terminal of the pre-synaptic neuron 200 opens a calcium channel 204 and allows an influx of Ca2+ ions into the plasma membrane through the calcium channel. The intracellular Ca2+ ions bind to vesicles 206 filled up the neurotransmitters 208 and cause the vesicles 206 to fuse into the plasma membrane for releasing the internal neurotransmitters 208 into the synaptic cleft 402. The released neurotransmitters 208 diffuse to flow across the synaptic cleft 402 and arrive at dendrite membranes of the post-synaptic neuron 100.
Here, the neurotransmitters 208 enable the stimulation transmitted from the pre-synaptic neuron 200 through two kinds of channels to be chemically transmitted into the post-synaptic neuron 100.
Exactly, one is a ligand-gated ion channel that uses the diffused neurotransmitter 208 as a ligand which directly binds to the ion channel. Namely, if the neurotransmitter 208 binds to Na+ channel 102, Na+ ion flows into the post-synaptic neuron 100 for contributing towards excitation and if the neurotransmitter 208 binds to K+ channel 104, K+ ion flows out of the post-synaptic neuron 100 for suppressing excitation.
The other is a G-protein coupled receptor 106 mediated ion channel that is activated by the diffused neurotransmitter 208 which directly binds to the G-protein coupled receptor 106 on the plasma membrane of a dendrite in the post-synaptic neuron 100. In this time, an alpha subunit of the G-protein coupled receptor 106 is dissociated and directly couples to the ion channel or indirectly couples to an effecter 108 on the inner membrane for operating this ion channel through an intracellular second messenger (not shown). In other words, if the second messenger couples to a Na+ gate 102, Na+ ion flows into the post-synaptic neuron 100 for contributing towards the excitation and if the second messenger couples to a K+ gate 104, K+ ion flows out from the post-synaptic neuron 100 for suppressing the excitation.
The intracellular Na+ ions flow in the dendrite membrane of the post-synaptic neuron 100 through the Na+ channels 102, diffuse across the cell body and then collect at the axon hillock 122. When the sum of the intracellular Na+ ions and the ions transmitted from other dendrites 120 induces the depolarization by a membrane potential more than the threshold value (Vth) at the axon hillock 122, a fire is produced as a spike signal shown in FIG. 2. The spike signal is an electrical signal for transmission of the stimulation by again repeating the depolarization and the repolarization along the axon 130 of the post-synaptic neuron 100.
In FIG. 1, the stimulation is transmitted from the pre-synaptic neuron 300 to two different dendrites 120 of the post-synaptic neuron 100 through two different synapses by two signals (a) and (b), respectively and can be fired when the sum (a+b) of two signals (a) and (b) is exceeded over the threshold value (Vth) at the axon hillock 122 of the post-synaptic neuron.
In FIG. 2, when the membrane potential reaches the threshold value (Vth, −55 mV) at the point {circle around (1)}, the membrane of the axon 130 of the post-synaptic neuron 100 opens the Na+ channels, which allow Na+ ion inflow to produce a fire by a sudden membrane potential rising and then, at the point {circle around (2)}, closes the Na+ channels and simultaneously opens the K+ channels, which allow K+ ion outflow to reduce the membrane potential until the K+ channels are closed at about −80 mV, and then maintains −70 mV of the resting (equilibrium) membrane potential by operations of Na+ pumps and K+ pumps.
By the above mentioned reasons, the first fire is mainly generated at the axon hillock 122 of the post-synaptic neuron 100 and the Na+ ions entered by the first fire are rapidly diffused by the myelin sheath 132 to depolarize the neighbor axon membrane. As a result, the spike waveform as shown in FIG. 2 is transmitted to the axon terminal.
On the other hand, although depending on the kind of a living body, the nervous system of a living body is consisted of enormous neurons. For example, the brain nervous system of a human is consisted of about 1011 neurons and each neuron is connected to about 1,000 synapses. Thus, the human brain nervous system is consisted of about 1014 synapses (Larry R. Squire, Eric R. Kandel, “MEMORY From Mind to Molecules”, Roberts & Company, 2nd ed., p. 30).
Therefore, in order to embody the neuromorphic computation system, it is indispensable to efficiently emulate the operating property of the enormous neurons.
Especially, it is very important to precisely and efficiently embody a firing process occurred in the axon hillock 122 of each neuron.
Until now, a representative model among the models of integrate-and-fire neuron circuits for emulating the firing process in the axon hillock 122 is the Mead's Axon-Hillock model (R. Sarpeskar, L. Watts, and C. Mead, California Institute of Technology, CA, CNS Tech. Rep. 1992).
By the way, because the Mead's model and the conventional most integrate-and-fire neuron circuits based on the Mead's model are using capacitors to accumulate the signals, all of the area, the power consumption and the delay time are increased to mimic each neuron. Especially, it is very difficult to implement the neuromorphic computation system consisted of the enormous neurons in a single chip.